How much ATP would a cell produce when using anaerobic respiration with nitrate in a 7 mM glucose solution? Assume 2.8 ATPs are produced per NADH and 1.9 ATP per FADH2. In your answer, total up all the NADH and FADH2 (if any) and all the ATP produced by Substrate Level Phosphorylation (SLP) and Oxidative Phosphorylation (OP). NADH FADH2 OP ATP SLP ATP Total ATP
The Correct Answer and Explanation is:
To determine the ATP yield, let us walk through the relevant processes involved in anaerobic respiration with nitrate as the terminal electron acceptor.
Glycolysis and Associated Energy Yield
- Glucose input: 1 molecule
- NADH produced: 2 from glycolysis
- FADH2 produced: 0 in glycolysis
- ATP from substrate-level phosphorylation (SLP): 2 ATP
Since nitrate serves as an alternative electron acceptor in place of oxygen, the electron transport chain remains operational in a modified form. This means oxidative phosphorylation can still occur, though it typically yields less ATP than in aerobic respiration. We are given the ATP equivalents per electron carrier:
- 2.8 ATP per NADH
- 1.9 ATP per FADH2
From glycolysis:
- NADH yield = 2
- FADH2 yield = 0
- OP ATP = 2 NADH × 2.8 = 5.6 ATP
- SLP ATP = 2
- Total ATP = 5.6 + 2 = 7.6
Final Table:
| NADH | FADH2 | OP ATP | SLP ATP | Total ATP |
|---|---|---|---|---|
| 2 | 0 | 5.6 | 2 | 7.6 |
Explanation:
Anaerobic respiration with nitrate as the terminal electron acceptor permits continued energy extraction beyond glycolysis. Glucose breakdown through glycolysis provides 2 ATP directly via substrate-level phosphorylation and yields 2 NADH. In organisms that utilize nitrate reduction, these NADH molecules transfer electrons through a modified electron transport chain, facilitating proton pumping and ATP synthesis via oxidative phosphorylation.
While FADH2 is commonly produced in the Krebs cycle, this cycle is often limited or bypassed in strict anaerobic conditions, especially if pyruvate is not further oxidized. As a result, no FADH2 is available to contribute ATP in this case.
The combined yield is modest compared to aerobic respiration, yet still efficient enough to sustain microbial life in anaerobic environments. A glucose concentration of 7 mM implies this process is scaled up proportionally in vivo, but the per-molecule ATP yield remains constant.
